Studies of the physical, chemical, and biological conditions of Toolik Lake and nearby lakes and ponds began in 1975. Since then an extensive series of manipulative experiments, multi-year fertilization treatments conducted in large mesocosms in Toolik, whole lake fertilization, and a lake trout removal experiment have expanded our understanding of the biogeochemical and ecological processes of arctic lakes. Toolik Lake serves as our primary long term monitoring site for lake systems. Thermal structure, nutrient, and major ion measurements have been made in the water column for the past 30 years. This long time-series of limnological conditions provides a basis for assessing the effects of environmental change expected in northern Alaska.